This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-371762, filed Dec. 27, 1999; and No. 2000-351612, filed Nov. 17, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates generally to a radio communication apparatus using an adaptive antenna, which is applied to a mobile radio communication system, etc., and more particularly to a transmission beam control apparatus for desirably controlling antenna directivity at the time of transmission.
In land mobile communications, there will frequently occur degradation in received signal level due to fading, or a signal distortion due to co-channel interference (CCI) or intersymbol interference (ISI).
In such a severe environment for signal propagation, in order to exactly extract a desired wave, it is effective to use an adaptive antenna which suitably controls antenna directivity. Adaptive arrays are well-known as an example of adaptive antennas. Adaptive arrays possibility suppress interface signals by pointing nulls to interfacing stations.
In land mobile communications, however, the size of a terminal station needs to be reduced for portability. In general cases, adaptive arrays are applied to a base station because of having plural antennas.
Adaptive arrays are array antennas which synthesize received signals from a plurality of antennas by controlling the phases and amplitudes of the signals. Even when high-level interference waves are present, a beam is turned to a direction from which a desired wave is incident and nulls (a point with no gain) are turned to directions from which interference waves are incident. Thereby, a reception SIR (a desired wave to interference wave ratio) can be increased to a maximum.
The operation of controlling and synthesizing the phases and amplitudes of received signals is equivalent to an operation wherein received signals from plural antennas #1, #2, . . . , #N are subjected to complex weighting, as shown in FIG. 1, by means of complex multipliers 1-1, 1-2, . . . , 1-N, which multiply N received signals by reception weight vectors calculated by a reception weight vector calculation section 2, and the resultant signals are synthesized by an adder 3. In this case, an output y (xe2x80x9cadaptive array outputxe2x80x9d) from the adder 3 is given by
y=wTxxe2x80x83xe2x80x83(1)
In this equation, w is a complex weight vector (hereinafter referred to as xe2x80x9creception weight vectorxe2x80x9d) applied to the received signal from each antenna, and x is a complex received signal vector from each antenna, w and x being expressed by
w=(w1, w2, . . . , wj, . . . , wN)Txe2x80x83xe2x80x83(2)
x=(x1, x2, . . . , xj, . . . , xN)Txe2x80x83xe2x80x83(3)
wherein T denotes transposition of matrix.
The reception weight vector w is controlled so that the adaptive array output y may satisfy a predetermined criterion. According to criteria, for example, an average square error between the adaptive array output y and an ideal signal sequence is reduced to a minimum, or a signal power of the adaptive array output y is reduced to a minimum under a constraint on the direction of arrival (DOA) of a desired wave. In this way, with respect to the reception in the base station (that is uplink), the received signals from plural antennas are weighted and consequently a distortionless signal can be obtained.
The above description is directed to the case where a omni-directional antenna is used as antenna, and this type of signal processing is called xe2x80x9celement space processing.xe2x80x9d On the other hand, there is known xe2x80x9cbeam space processingxe2x80x9d wherein a plurality of beams with different directions of radiation are formed in advance and received signals obtained by the beams are subjected to an adaptive array processing.
If a beam space adaptive array is used, a pre-processing beam generator needs to be additionally provided. However, since a signal output with a high SNR (signal-to-noise ratio), combined with a beam gain, is obtained, a stable adaptive array processing can be expected by selecting appropriate beams. Moreover, since the number of branches input to the adaptive array can be reduced, the amount of arithmetic operations for signal processing can be reduced accordingly.
This feature is described, for example, in document [1] (Chiba, Nakajo, Fujise, xe2x80x9cBEAM SPACE CMA ADAPTIVE ARRAY ANTENNAxe2x80x9d, IEICE Transaction of the Communications, B-II, vol. J77-B-II, no. 3, pp. 130-138, March 1994).
In general, a beam space adaptive array generates spatially orthogonal beams. However, as non-orthogonal beams, an adaptive antenna using directional antennas overlapping between adjacent beams has been studied.
For example, document [2] (Jpn. Pat. Appln. KOKAI Publication No. 10-256821 (Matsuoka, et al.)) proposes an adaptive antenna capable of efficiently combining delay wave energy by performing not only a space domain process using an adaptive array antenna but also a time domain process using path diversity.
On the other hand, many downlink beam forming methods to synthesis optimal transmission beam pattern using an array antenna has been studied. For example, in TDD (Time Division Duplex) system wherein transmission/reception is periodically switched by time division, since the same frequency is used in the transmission/reception, it can be regarded that the propagation channel responses of transmission and reception are substantially equal.
Accordingly, as shown in document [3] (Tomisato, Matsumoto, xe2x80x9cEFFECT OF ADAPTIVE TRANSMISSION ARRAY IN TDD MOBILE COMMUNICATION SYSTEMxe2x80x9d, 1997 IEICE Spring conference, B-5-87, March 1997), the reception SIR at the terminal station can be improved by using the same weight vector for transmission/reception, i.e., by forming the same antenna pattern in transmission as is obtained at the time of reception.
However, as in the case of FDD (Frequency Division Duplex) where different frequencies are used for transmission and reception, the correlation in propagation channel response between uplink and downlink is small. Thus, even if a transmission weight vector that is equal to a reception weight vector is used, optimal reception at the terminal station is not always ensured (e.g. see document [4] (J. Litva, T. K.-Y. Lo, xe2x80x9cDigital Beamforming in Wireless Communications,xe2x80x9d Artech House Publishers, pp. 182-183, 1996).
As stated above, although the propagation channel response differs between uplink and downlink, there is reversibility between uplink and downlink with respect to the direction of arrival of radio waves. Specifically, except for a case where the speed of movement of the terminal station is excessively high, the reception SIR at the terminal station can be increased to a maximum by estimating DOA of reception ratio waves at the base station and setting the beam and null in that direction.
For the purpose of such transmission beam pattern control, the estimation of the DOA is indispensable. As a signal processing for the estimation of the DOA, there is known a MUSIC (MUltiple SIgnal Classification) algorithm, etc. are known.
However, a great amount of calculations is required for a high-resolution DOA estimation algorithm represented by MUSIC. This algorithm is not suitable in a case of estimating the DOA which varies from time to time depending on the movement of the terminal station or a variation in environment.
Even if numerous arithmetic operations are performed to precisely estimate the DOA, and the weighting for directivity is carried out to set the null in the estimated DOA, the direction of the null may deviate due to defective calibration of the transmission circuit. Furthermore, the effect of this technique may deteriorate due to the angle spread by reflection/dispersion near the terminal station in the actual propagation path. As a result, the average reception SIR at the terminal station may deteriorate.
Besides, if the number of incoming waves is greater than the number of antennas in the multi-path environment, it is difficult to estimate the DOA by MUSIC.
The present invention has been made to solve the above problems, and its object is to provide a radio communication apparatus which is applicable to a system using different frequencies for uplink and downlink and can easily estimate a DOA of radio waves and enhance an average reception SIR at an opposing-side station.
In order to solve the above problems, the present invention has the feature that an arrival angle range indicating an approximate arrival direction of a desired wave is estimated on the basis of a delay profile estimated in a reception system for a predetermined signal processing (e.g. a temporal/spatial equalization signal processing), and an optimal antenna or beam is selected on the basis of the arrival angle range, thereby effecting signal transmission.
Specifically, a radio communication apparatus according to the present invention comprises: a plurality of directional antennas; a delay profile estimation section configured to estimate a delay profile representing arrival times of a desired wave and delay waves and received powers for each of received signals from said antennas; an arrival angle range estimation section configured to estimate an arrival angle range of said desired wave from the delay profiles of the received signals estimated by said delay profile estimation section; a transmission antenna selection section configured to select at least one of the antennas which is to be used for transmission, on the basis of the arrival angle range estimated by said arrival angle range estimation section; and a transmission section configured to transmit transmission signals using said at least one antenna selected by said transmission antenna selection section.
With the above structure, in a mobile radio communication system using different frequencies for uplink and downlink, an arrival angle range of a desired wave can easily be estimated only by observing a received power value of each of the desired wave and delay waves, making use of a delay profile for each directional antenna which has already been measured for a temporal/spatial equalization processing. It is possible to easily select an optimal transmission antenna based on a radio wave arrival direction in which reversibility is established between uplink and downlink. An average reception SNR and an average reception SIR can be enhanced at an opposing-side station. Since an arrival angle range can be detected for delay waves at the same time, a transmission (time) diversity effect is expected to be obtained by using the DOA for delay wave.
In the above basic structure, the radio communication apparatus may further comprise arrival direction estimation section configured to estimate a direction of arrival of the desired wave from the arrival angle range estimated by the arrival angle range estimation section; and transmission weight vector generating section configured to generate such transmission weight vectors as to set a maximum gain direction of directivity at a time of transmission in the estimated arrival direction.
In this case, the transmission antenna selection section selects a plurality of antennas included in the arrival angle range estimated by the arrival angle range estimation section, and the transmission section feeds to the antennas selected by the transmission antenna selection section transmission signals multiplied by the transmission weight vectors generated by the transmission weight vector generating section, and transmits the transmission signals. Thereby, the maximum gain direction of the transmission beam can be made to coincide with the desired wave arrival direction, and the average reception SNR can be maximized at the opposing-side station.
Another radio communication apparatus according to the invention comprises: a plurality of antennas disposed in a predetermined shape; beam forming section, connected to each antenna, for forming a plurality of beams having different directions of radiation; a plurality of delay profile estimation section each configured to estimate delay profiles representing arrival times of a desired wave and delay waves and received powers for each of received signals by the formed beams; arrival angle range estimation section configured to estimate an arrival angle range of the desired wave from the estimated delay profiles; transmission beam selection beams for selecting at least one of the beams which is to be used for transmission, on the basis of the estimated arrival angle range; and transmission section configured to transmit the transmission signals using the selected beam.
With this structure, like the above-describe one, an arrival angle range of a desired wave can easily be estimated by making use of a delay profile. It is possible to easily select an optimal transmission antenna based on a radio wave arrival direction in which reversibility is established between uplink and downlink. An average reception SNR and an average reception SIR can be enhanced at an opposing-side station.
In this basic structure, the radio communication apparatus may further comprise: arrival direction estimation section configured to estimate a direction of arrival of the desired wave from the arrival angle range estimated by the arrival angle range estimation section; and transmission weight vector generating section configured to generate such transmission weight vectors as to set a maximum gain direction of directivity in the estimated arrival direction.
In this case, the transmission beam selection section selects a plurality of beams included in the arrival angle range estimated by the arrival angle range estimation section, and the transmission section reflects on the beams selected by the transmission beam selection section transmission signals multiplied by the transmission weight vectors generated by the transmission weight vector generating section, and transmits the transmission signals. Thereby, the maximum gain direction of the transmission beam can be made to coincide with the desired wave arrival direction, and the average reception SNR and average reception SIR can be maximized at the opposing-side station.
In an embodiment, in the arrival direction estimation section, the arrival direction is detected by performing a scan using a predetermined scanning beam pattern within the arrival angle range estimated by the arrival angle range estimation section, and finding a maximum value of a reception output level obtained by the scan. Thereby, it is possible to easily estimate the arrival direction necessary for forming the transmission beam pattern for effecting maximum-gain radiation in the desired DOA.
In another embodiment of the arrival direction estimation section, the arrival direction is detected by performing a scan using a predetermined scanning null pattern within the arrival angle range estimated by the arrival angle range estimation section, and finding a minimum value of a reception output level obtained by the scan. Compared to the scan using the scanning beam pattern, the desired wave arrival direction can be estimated more precisely.
In still another embodiment of the arrival direction estimation section, the above two section are combined, and the arrival direction is detected by performing a scan using a predetermined scanning beam pattern and a scan using a predetermined scanning null pattern within the arrival angle range estimated by the arrival angle range estimation section, and finding a maximum value in difference between reception output levels obtained by the both scans. According to this method, compared to the arrival direction estimation method using only one of the beam pattern scanning and null pattern scanning, the error in estimation of the arrival direction can be reduced.
The transmission section may effect transmission by forming a transmission beam pattern which has a maximum gain in the arrival direction of the desired wave and has suppressed side lobes in directions other than the arrival direction of the desired wave. In this case, it is not always possible to maximize the average reception SIR at the opposing-side station. However, the beam capable of achieving high SIR can be formed even in a case where the arrival direction of interference waves varies over time. Moreover, since there is no need to use the high-resolution arrival direction estimation method for the null control, the amount of arithmetic calculations can be remarkably reduced.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by the instrumentalities and combinations particularly pointed out hereinafter.